1. Field of the Invention
This invention relates to high-capacity radial fit coupling bolts. (Such bolts are also known as “expanding sleeve bolts”.)
The invention particularly relates, but is not limited to, high-capacity radial fit coupling bolts used to secure engineering components securely together. Examples of potential uses for the bolts include connecting pipeline flanges, drive couplings, machinery mountings and the like.
2. Prior Art
NB: The following discussion is by way of background information only, and is not to be considered a statement of the common general knowledge (CGK) in the area of technology.
When establishing the specifications of drive couplings for power transmission shafts, engineers calculate the size of the bolts needed by formulae which consider factors such as: (a) coupling diameter, (b) power throughput and (c) materials used to predict bolt size, frequency and tensile load requirements. Power transmission is ideally made via the frictional grip between opposing coupling faces, but will usually also consider the resistance to shear forces which may be directed against the bolts' shanks during operation. Where the shear forces may occur as a result of vibration, slippage and/or lack of bolt tension, the impingement of coupling bolt holes against their respective bolt shanks often damages and distorts these components, making disassembly extremely difficult. It can be seen that the application and maintenance of the specified bolt tension in all the bolts is vital in maintaining the efficacy of the drive couplings. It can also be seen that a bolt which can expand radially to fill any void formed by the tolerance between bolt shank and its respective bolt holes in the drive coupling will substantially assist limiting any slippage of the coupling faces.
For some years it has been common practice to employ bolts which have this capacity. In currently used types, which are commonly referred to as “Radial Fit Coupling Bolts”, the bolt component is manufactured having an enlarged, tapered portion at the midsection of the shank with screw-threaded portions at each end. The tapered section of the bolt is received within a complementary-shaped tapered sleeve which, when placed in position through aligned bolt holes in the drive coupling components to be clamped, is diametrically expanded by the action of drawing the tapered portion of the shank into the tapered sleeve. This diametrical expansion will continue in response to the applied force until the sleeve completely fills the bolt holes through the coupling components.
Typically in such a bolt, the external tapered surface of the central portion of the shank and the inner surface of the sleeve must be manufactured to a very fine finish, and may require the surfaces to be polished. In addition, a ‘track” may be machined in the tapered surface of the central portion of the shank. The inner taper may become locked inside the external sleeve and difficult to remove, requiring the injection of oil under pressure to eject the bolt and to allow the sleeve to collapse; and as the sleeve is diametrically expanded, the portions overlying the “track” do not expand to the same extent. When the components secured by the bolt are to be released, the sleeve may jam about the central portion of the shank and remain in relatively high frictional engagement with the aligned bolt holes in the components, requiring considerable force e.g. applied by a press, to release the bolt from the bolt holes.
The manufacturers of these known bolts recommend that a back nut be fitted to the screw-threaded portion at one end of the bolt shank (to engage one of the coupling components); a tensioning load applied to the other end of the bolt shank; and, when the desired expansion of the sleeve is achieved, the tensioned other end of the shank may then be locked in place by tightening down a nut on that end, the nut engaging the second of the coupling components.
Such designs exhibit a clear disadvantage in that the original “setting” of the sleeve will lock the bolt in place via sticking friction. Thus, when the bolt is tensioned with the back nut in place, very little of the applied tensile load will migrate past the tapered section of the shank to tension the opposite end thereof and lock the back nut. This may be redressed by tightening the shank from that end also, but will have the effect of decreasing the “setting” force which occasions the diametrical expansion of the sleeve.